One of the major problems of large capacity turbo-machines as described above relates to the vibration that such machines can generate, in particular at the bearings. In most cases, the origins of such vibration stem from rotor unbalance. For industrial fans of large capacity, e.g. of the order of 100 kW to 10,000 kW, their normal operating conditions necessarily imply that an unbalanced state of the rotor appears gradually or suddenly.
Such appliances are designed to provide ventilation in industrial fields as varied as the nuclear industry, the chemical industry, iron and steel works, cement making, or indeed in fossil fuel power stations.
Consequently they are commonly exposed to the risk of particles or blown matter being deposited on or by the blades of the fan, to progressive wear of the blades of the fan due to the various corrosion effects of the fluids or gases passing through the fan, or indeed to deformation due to sudden temperature variations.
The deposition of matter (clogging) on the blades of the fan also generally takes place in gradual and non-uniform manner on the various blades of the fan, thereby giving rise to a first rotor-unbalancing effect. In certain applications, for example in cement works, this first rotor-unbalancing effect can be accompanied by a second unbalancing phenomenon that is much more sudden, being caused by one or more blocks of material that were previously stuck to the blades becoming detached therefrom suddenly. This second phenomenon gives rise to large and violent unbalance of the rotor which is particularly dangerous for large-capacity appliances in which blade diameter can easily be three meters (m) and the speed of rotation at least 1000 revolutions per minute (rpm).
For all of the situating mentioned above by way of non-limiting example, the appearance of vibration well above the operating limit of an appliance means that the fan must be brought to rest quickly, and consequently that all or part of an industrial or production facility must also be stopped while the blades are being cleaned or the rotor is being balanced. In addition, the operations of cleaning or rebalancing the rotor are lengthly and particularly difficult. Industrially and economically speaking, the downtime of the fan represents a large expense that is difficult to accept.
There is thus a great need for a solution that is practical, effective, and quick to the above-mentioned problems of turbo-machines such as industrial fans.
Proposals have already been made to implement moving-weight, dynamic balancing apparatus for an industrial fan that is capable of continuously and automatically monitoring the unbalance/balance state of the fan in operation, and also of continuously performing the necessary balance-correcting operations. Such apparatus makes use of at least one balancing unit, specifically a ring, that is carried by the shaft supporting the rotor, i.e. the fan wheel. The balancing ring includes, as balancing means, a high density fluid capable of vaporizing quickly in contact with heater devices installed in the fluid flow circuit. The system for monitoring and correcting the unbalance state of the rotor includes a microprocessor, measures the level of fan vibration, and determines the location of the unbalance.
The monitoring and correction system then feeds electricity to the heater device situated in the vicinity of the place where the unbalance has been found, so as to cause the fluid situated in this zone to be vaporized. Under such circumstances, the vaporized fluid is transferred into a cooling chamber situated in an opposite zone where it recondenses and thus returns to its initial liquid state. This fluid transfer phenomenon serves to balance the rotor.
Such apparatus provides a genuine contribution to controlling rotor unbalance, however it nevertheless suffers from a certain number of drawbacks associated in particular with the impossibility of avoiding the particularly tough and extreme operating conditions in which industrial fans must necessarily operate. Thus, the extreme temperature variation conditions to which industrial fans are subject can have a negative effect on the accuracy with which balancing or rebalancing of industrial fan rotors is performed when balancing is done by means of fluid vaporization/condensation. Also, since the correction of fan unbalance relies on transferring fluid from a hot point to a condensation cold point, the accuracy with which such apparatus can be controlled is limited because of the similarly limited number of cooling chambers around the periphery of the ring. In other words, rotor rebalancing is performed at discrete balancing zones at the periphery of the balancing ring, and not at any point chosen continuously around the entire periphery of the ring, thereby leading to relatively inaccurate balancing.
Finally, it should be observed that a balancing system making use of a fluid circuit is relatively fragile since the circuit must be perfectly fluid-tight even though the usual operating conditions of industrial fans imply that ambient conditions are particularly difficult and corrosive. It also turns out that the balancing capacity of fluid systems is limited to industrial fans of small capacity.
In a technical field different from that of industrial fans, and specifically for machines requiring small balancing capacity, namely grinding machines, it is already known to use balancing rings and an unbalance detection system which includes a microprocessor controlling the displacement of the rings. In that system, the balancing unit is internally mounted inside the body itself of the grinding wheel. Such solutions are not suitable for balancing large masses or for operating in difficult environmental conditions.